Organotins induce apoptosis by disturbance of [Ca2+](i) and mitochondrial activity, causing oxidative stress and activation of caspases in rat thymocytes

Di-n-butyltin dichloride (DBTC) and tri-n-butyltin chloride (TBTC) cause thymus atrophy in rodents. At low doses, antiproliferative modes of action have been shown to be involved, whereas at higher doses apoptosis seems to be the mechanism of thymotoxicity by these chemicals. In vitro, a similar concentration-dependency has been observed. The purpose of the present research was to investigate the mechanisms underlying DNA fragmentation induced by these organotin compounds in freshly isolated rat thymocytes. As previously shown for TBTC, DBTC is also able to significantly increase intracellular Ca2+ level ([Ca2+](i)). The rise in [Ca2+](i), already evident 5 min after treatment, was followed by a dose- and time-dependent generation of reactive oxygen species (ROS) at the mitochondrial level. Simultaneously, organotins induced the release of cytochrome c from the mitochondrial membrane into the cytosol. ROS production and the release of cytochrome c were reduced by BAPTA, an intracellular Ca2+ chelator, or rotenone, an inhibitor of the electron entry from complex I to ubiquinone, indicating the important role of Ca2+ and mitochondria during these early intracellular events. Furthermore, we demonstrated that rotenone prevents apoptosis induced by 3 μM DBTC or TBTC and, in addition, that both BAPTA and Z-DEVD FMK (mainly a caspase-3 inhibitor) decreased apoptosis by DBTC (already shown for TBTC). Taken together these data show the apoptotic pathway followed by organotin compounds starts with an increase of [Ca2+](i), then continues with release of ROS and cytochrome c from mitochondria, activation of caspases, and finally results in DNA fragmentation